Over the last decades, magnetron sputtering has become a well-known technique to deposit thin coatings such as metal coatings or ceramic coatings on large area substrates such as glass or elongated flexible substrates.
The technique of sputtering is typically used to deposit silicon oxide, silicon nitride, titanium oxide, zinc oxide, tin oxide, . . . .
The sputter deposition rate and the degree of target consumption are important issues to make the sputter process economically interesting.
By the introduction of the rotatable sputter target, the sputter process became more attractive as several advantages can be realized. The most important advantages are the higher target material consumption, the possibility to use a higher power density and thus to obtain a higher sputter deposition rate, the enhanced anode functionality during AC sputtering and the drastically reduced arc sensitivity during reactive sputtering.
However, while realizing a number of advantages, the rotatable sputter target also shows problems.
Reactive sputtering involves working and controlling the sputter deposition rate of a metal in a reactive gas as known from the hysteresis curve.
High sputter deposition rates are obtained in the metallic mode.
However, as the reactive gas may interact with the target material, an insulating material can be built up at all exposed a non-sputtering surfaces.
To obtain higher sputter deposition rates, one has developed systems with fast feedback loops to sustain the target in a metastable semi-poisoned state. While monitoring sensitive signals of the process (e.g. partial pressure, target voltage, optical emission of the plasma, . . . ) one can feedback any drift from the unstable situation to maintain the semi-poisoned target state.
Although the consumption of a rotatable target is significantly higher compared to planar targets, still a considerable amount of target material is lost by the setup of the sputter target and the magnet array and the consumption of the target material of a rotatable target remains an issue.
During operation, the rotatable sputter target rotates below the stationary race track. At the race track turns a higher power density and thus a higher sputter deposition is obtained, leading to a higher consumption of the target material. Once all the available target material is consumed at the race track turns, the target has to be replaced although still an appreciable amount of valuable material may be present over the main part of the target.
To overcome these problems, it is known in the art to apply more material at the ends of the sputter target. The sputter targets obtained in this way are called dog-bone targets.
However, material brittleness and manufacturing problems limit the extent to which the dog-bone solution can be used.
Although the above-mentioned problems could be solved by using an adequate feedback loop system and by using targets having a dog-bone structure, new problems arise. The target consumption drops for example to values typically known for planar targets.
U.S. Pat. Nos. 5,470,452 and 5,725,746 disclose rotatable targets comprising an elongated tubular member having a target material at the outer surface thereof. The target comprises a collar of an electrically conductive material at least one end of the tubular member. The target material is substantially free of the collar material. The collar material suppresses arcing. However, this type of rotatable targets shows a number of drawbacks.
As the thermal contact between the collar and the target material is not optimal, thermal stresses in the collar material can be high.
The collar will be thermally expanded to a higher extent compared to the rest of the target material. Consequently, the thermal contact between the collar and the water cooled target material will be further reduced. Finally, during sputtering the collar will function as a loose glowing ring over the rotating target material and may cause local melt zones on the target material.
A further drawback of this type of rotatable targets is that material sputtered from the collar material will be deposited also on the edges of the substrate being coated where it will be mixed with material sputtered from the target.